Acids in HF

Hydrogen fluoride is a weak acid in dilute solution in water but H0 measurements (Bell et al., 1956) show that in more concentrated solutions the acidity rises rapidly so that 100% HF has an H0 value only slightly less than that of 100% H2S04. Confirmation of this comes from cryoscopic measurements which show that the weak bases water, acetonitrile and p-nitrotoluene are somewhat less ionized in 100% HF than in 100% H2S04 (Gillespie and Humphreys, 1970). 

The exact value of H0 for pure 100% HF is uncertain as it appears to depend strongly on the presence of small amounts of impurities which are very difficult to remove the most important of these is water which exhibits weak basic behaviour. Thus the measured values of —H0 increase from 9-7 to 11-0 with decreasing impurity concentration, as indicated by the electrical conductivity (Hyman and Katz, 1965). The value of —11 0 was obtained with an acid that had a conductivity of 3 x 10-B ohm-1 cm-1 at 0°. Presumably, if the last traces of water and other impurities could be removed from HF, the value of — H0 would be greater than 11-0, perhaps as high as 11 -5. A few measurements have been made on solutions of sodium fluoride, which causes a marked decrease in — H0 and on a 0-02m solution of NbF6, which had H0 —12-5 (Hyman et al., 1961). 

It has been shown from cryoscopic (Dean et al., 1970) and conducti-metric (Gillespie and Moss, 1966) measurements that SbF6 is a strong acid in HF ionizing in dilute solutions according to equation (13). 

At higher concentration of SbF6 the anions Sb2Fji (5), Sb8Fjj, (6), etc. are formed with increasing concentrations of SbFB. Presumably, therefore, there is very little increase in acidity when the formation of these ions predominates over the formation of SbFjf. 

Solutions of arsenic pentafluoride have conductivities and freezing point depressions (Dean et al., 1970) that are only approximately one-half those of SbF5 at the same concentration and it has been shown that even in dilute solutions there is essentially complete formation of the As2Ff1 ion according to equation (14). 

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The X-ray crystal structure of protonated formic acid, acetic acid, and methyl formate has recently been determined by Minkwitz and co-workers. In agreement with NMR data discussed above, the nearly equal C—O bond lengths of protonated formic acid (1.239 and 1.255 A),573 protonated acetic acid 275 (1.251-1.291 A for various salts),574 and protonated methyl formate 276 (1.260 and 1.264 A)575 show efficient delocalization of the positive charge. Minkwitz et al.576 have also studied protonation of oxalic acid in HF-SbF5 and isolated the hexafluoroantimonate of the mono- and diprotonated acid at 75°C and 40°C, respectively. Structural characteristics are very similar to those of the other cations discussed. 

The available physico-chemical evidence suggests that PF5 is a vanishingly weak Lewis acid in HF. It was shown in Sec. 11.1.2 that the value of —15.1 for Ho for pure HF was obtained by interpolation between measured values of H0 for HF solutions of strong Lewis acids and for HF containing strong base. Neat HF, the working solvent after distillation, usually gives experimentally measured Ho values of about —11, because of the presence of very small traces of water... 

Summary. There is now ample physico-chemical evidence for the ordering of the relative strengths of the fluorides presented in Secs 11.2.3.1.1 to 11.2.3.1.4 as Lewis acids in HF. There has been much less quantitative investigation of other pentafluorides and oxidefluorides, such as ReF5 and WOF4, but the information that is available [4(b)] indicates an order ... 

Subsequently Bartlett and colleagues [72,73] made detailed studies of the oxidant strengths of the anions AgF4 and NiF -, of the neutral binary fluorides AgF3 and NiF4, derived from the anions by action of Lewis acids in HF, and of cationic... 

Kubisova L, Sabolova E, Schlosser S, Martak J, Kertesz R. Mass-transfer in membrane based solvent extraction and stripping of 5-methyl-2-pyrazinecarboxyhc acid and co-transport of sulphuric acid in HF contactors. Desalination 2004 163 27-38. 

Potassium fluotantalate or tantalofluoride, K2TaF7, is formed by dissolving tantalic acid in HF, and adding the correct amount of KF. It is sparingly soluble in cold water but dissolves quite readily in hot water. It crystallizes from solution in fine rhombic needles whieh may be melted without decomposition. When a solution of the fluotantalate is boiled, a white insoluble oxy-fluoride of the composition 4 KF 2 TaFe Ta20 8 is precipitated. Columbium does not behave in a similar manner, consequently this reaction is used to detect small quantities of tantalum in columbium material. 

In addition, the presence of CIF2 was identified in solutions of CIF3 itself in HF or BrFa without the addition of any outside Lewis acid > ). In HF solution an ionization process takes place according to... 

Dibenzyl formylation was carried out by Tanaka et a/. Various Lewis acids in HF were used to investigate the influence of acidity on regioselectivity. The order of acidity are SbF5>TaF5>BF3>NbFs and the results displayed in Table 1.10 indicate that both the ortho regioselectivity and aldehyde yield decreased with a decrease in Lewis acidity. 

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